Interest in thin-film photovoltaics has expanded in recent years. This is due primarily to improvements in conversion efficiency of cells made at the laboratory scale, and the anticipation that manufacturing costs can be significantly reduced compared to the older and more expensive crystalline and polycrystalline silicon technology. The term “thin-film” is used to distinguish this type of solar cell from the more common silicon based cell, which uses a relatively thick silicon wafer. While single crystal silicon cells still hold the record for conversion efficiency at over 20%, thin-film cells have been produced which perform close to this level. Therefore, performance of the thin-film cells is no longer the major issue that limits their commercial use. The most important factor now driving the commercialization of thin-film solar cells is cost. Currently, a widely accepted technology solution for the scale up to low-cost manufacturing does not exist.
Attempts have been made and are now being made to remedy the problem, but progress has been slow. While a large infrastructure exists for the sputter coating of glass for the architectural window market, this process is not readily adapted to the production of solar cells for several reasons. First, the glass that is coated in large-scale machines is relatively thick compared to that used in solar modules. Also, the glass must be heated to temperatures far above that required in the window industry, causing large yield losses from fracturing and breakage. Handling large sheets of glass is expensive in terms of floor space and equipment, and the extra layers in a solar cell require additional large coating chambers with appropriate gas isolation between chambers. Finally, and maybe most importantly, efficient sputtering targets have not yet been made for the deposition of the absorber layer, which in many respects is the most challenging aspect of making a thin-film solar cell.
An early attempt to improve manufacturing of solar cells with a roll-to-roll technique was proposed by Barnett et al in U.S. Pat. No. 4,318,938 ('938) issued 9 Mar. 1982. They describe a roll-to-roll machine, which consists essentially of a series of individual batch processing chambers each adapted to the formation of a different layer. A thin foil substrate is continuously fed from a roll in a linear belt-like fashion through the series of individual chambers where it receives the required layers. Several of the layers are formed by evaporation of the desired material in vacuum chambers. The metal foil is transferred continuously from air to vacuum and back to air several times. The patent does not describe how this is accomplished, other than the statement that such technology can be purchased. Much has changed in recent years. The copper sulfide absorber layer proposed in '938 has been shown to be unstable in the field, and some of the other layers are no longer used. In particular, it is undesirable to have a pinch roller running on a newly formed coating layer. However, the inventors estimated that their continuous technique could reduce the manufacturing cost by as much as a factor of two over the conventional batch process for silicon. While a factor of two is still significant today, greater reductions in cost must be achieved if solar power is to become competitive with conventional sources of power generation.
Matsuda et al in U.S. Pat. No. 5,571,749 ('749) issued 5 Nov. 1996 teach a roll-to-roll coating system based on plasma chemical vapor deposition (CVD) techniques. Their system is a single linear vacuum chamber with a series of six gas gates for process isolation. The web substrate is passed through the machine in belt-like fashion similar to the method of '938, but the web remains in vacuum for the whole process. The solar cell absorbing layer is made from amorphous silicon deposited from the decomposition of silane gas. Different dopants are introduced along the belt path to create the required p-n junctions. Similar techniques are used at Uni-Solar of Troy, Mich. to make a variety of amorphous silicon solar cells. The conversion efficiency of amorphous silicon cells is inferior to that of the other thin film cells, and they suffer a loss of efficiency during the initial few weeks of exposure to solar radiation through a mechanism known as the Stabler-Wronski effect. Because of this the efficiencies of amorphous silicon remain well below that of other thin-film materials, and no one has yet found a way to mitigate the effect.
Wendt et al disclose a roll-to-roll system in U.S. Pat. No. 6,372,538 ('538) issued 16 Apr. 2002 that teaches a method for depositing a thin film solar cell based upon a copper indium/gallium diselenide (CIGS) absorber layer. The system is described as consisting of nine separate individual processing chambers in which a roll-to-roll process may be used at each chamber. Thus the overall system is similar to that described in '938, but without the continuous belt-like transport of the substrate through all of the chambers at once. Also the roll of thin material (polyimide in this case) is not continuously fed through a single vacuum system as it is in '749. Wendt et al teach conventional planar magnetron sputtering for the deposition of a molybdenum back contact layer onto the polyimide film. Adjustments are made to the argon gas pressure and some oxygen is introduced to adjust the film stress to accommodate the expansion of the polyimide when it is heated for the CIGS deposition. Incorporation of oxygen into the molybdenum layer increases its resistivity, requiring the layer to be thicker to provide adequate electrical conductivity. The CIGS materials are deposited over the molybdenum layer in a separate chamber using an array of thermal evaporators each depositing one of the components. The use of the polyimide substrate material presents at least two problems in processing. First, it contains a relatively large amount of adsorbed water, which is evolved in the vacuum system and can have negative effects on the process. And secondly, it cannot withstand the higher temperatures used for the deposition of high quality CIGS material. Thin foils of stainless steel would have neither of these problems. The preferred width of the polyimide web is 33 cm, and it runs at a typical line speed of 30 cm per minute. With respect to the present invention, such production rates (about a square foot per minute) are not considered large-scale; rather, rates 5 to 10 times faster with attendant cost reductions are necessary to make solar power competitive with power from conventional sources.
Copper indium diselenide (CuInSe2 or CIS) and its higher band gap variants copper indium gallium diselenide (Cu(In/Ga)Se2 or CIGS), copper indium aluminum diselenide (Cu(In/Al)Se2), and any of these compounds with sulfur replacing some of the selenium represent a group of materials that have desirable properties for use as the absorber layer in thin-film solar cells. The acronyms CIS and CIGS have been in common use in the literature for sometime. The aluminum bearing variants have no common acronym as yet, so CIGS is used here in an expanded sense to represent the entire group of CIS based alloys. To function as a solar absorber layer these materials must be p-type semiconductors. This is accomplished by establishing a slight deficiency in copper, while maintaining a chalcopyrite crystalline structure. Gallium usually replaces 20% to 30% of the normal indium content to raise the band gap; however, there are significant and useful variations outside of this range. If gallium is replaced by aluminum, smaller amounts of aluminum are required to achieve the same band gap.
CIGS thin-film solar cells are normally produced by first depositing a molybdenum (moly) base electrical contact layer onto a substrate such as glass, stainless steel foil, or other functional substrate material. A relatively thick layer of CIGS is then deposited on the moly layer by one of two widely used techniques. In the precursor technique, the metals (Cu/In/Ga) are first deposited onto the substrate using a physical vapor deposition (PVD) process (i.e. evaporation or sputtering), chemical bath, or electroplating process. Subsequently, a selenium bearing gas is reacted with the metals layer in a diffusion furnace at temperatures ranging up to about 600° C. to form the final CIGS composition. The most commonly used selenium bearing gas is hydrogen selenide, which is extremely toxic to humans and requires great care in its use. A second technique avoids the use of hydrogen selenide gas by co-evaporating all of the CIGS constituents onto a hot substrate from separate thermal evaporation sources. While the deposition rates are relatively high for thermal evaporation, the sources are difficult to control to achieve both the required stoichiometry and thickness uniformity over large areas of a substrate. Neither of these techniques for forming the CIGS layer is readily scalable to efficient large-scale production.
In part, moly is used as the back contact layer because of the high temperature required for the CIGS deposition. Other metals (silver, aluminum, copper etc.) tend to diffuse into and/or react with the selenium in the CIGS at the elevated deposition temperatures, and create an undesirable doping or interface between the contact layer and the CIGS layer. Moly has a very high melting point (2610 C), which helps to avoid this problem, although it will react with selenium at high temperatures. However, even if the reactive interface is minimized, moly still has a rather poor reflection at the interface with the CIGS layer, resulting in decreased efficiency since the light that penetrates the absorber initially is not reflected back through the CIGS effectively for a second chance at being absorbed. Therefore, replacing the moly with a better reflecting layer can allow a decrease in the thickness of the absorber layer as well as provide improved cell performance by moving the absorption events closer to the p-n junction.
The n-type material most often used with CIGS absorbers to form the thin “window” or “buffer” layer is cadmium sulfide (CdS). It is much thinner than the CIGS layer and is usually applied by chemical bath deposition (CBD). Cadmium is toxic and the chemical bath waste poses an environmental disposal problem, adding to the expense of manufacturing the cell. CBD zinc sulfide (ZnS) has been used successfully as a substitute for CdS, and has produced cells of comparable quality. The CBD method for ZnS is not as toxic as CdS; but, remains a relatively expensive and time-consuming process step, which should be avoided if possible. Radio frequency (RF) sputter deposition of CdS and ZnS has been demonstrated on a small scale. However, RF sputtering over large areas is difficult to control because the plasma is highly influenced by the chamber geometry in the conventional method of implementing RF sputtering. An improved method of RF sputtering. ZnS is needed to reduce the process complexity as well as to eliminate the toxic cadmium from the process.
Finally, the window or buffer layer is covered with a relatively thick transparent electrically conducting oxide, which is also an n-type semiconductor. In the past zinc oxide (ZnO) has been used as an alternative to the traditional, but more expensive, indium tin oxide (ITO). Recently, aluminum doped ZnO has been shown to perform about as well as ITO, and it has become the material of choice in the industry. A thin “intrinsic” (meaning highly resistive) ZnO layer is often deposited on top of the buffer layer to cover any plating flaws in the CdS (hence “buffer” layer) before the cell is completed by the deposition of the transparent top conductive layer. In order to further optimize the performance of the cell, an antireflection coating may be applied as a final step. Because of differences in refractive index, this step is more important for silicon cells than for CIGS cells in which some level of antireflection is provided by the encapsulation material when the cells are made into modules. In the case of CIGS an antireflection coating may be applied to the outer surface of the glass.
The difficulties inherent in the deposition of CIGS related absorber layers as well as the buffer layer have prevented these thin-film solar cells from being readily manufactured in large scale with improved economies and lower costs. Concurrent improvements in the back reflector and elimination of cadmium and its waste disposal problems can also lower the cost per watt of generated solar power.
A conventional prior art CIGS solar cell structure is shown in FIG. 1. Because of the large range in the thickness of the different layers, they are depicted schematically. The materials most often used for each of the layers are also indicated in the figure. The arrow at the top of the figure shows the direction of the solar illumination on the cell. Element 1 is the substrate, and it is massive in relation to the thin-film layers that are deposited upon it. Glass is the substrate that has been commonly used in solar cell research; however, it is more likely that for large-scale production some type of foil-like substrate will be used. Layer 2 is the back electrical contact for the cell. Traditionally, it has been moly with a thickness of about 0.5 to 1.0 microns. While moly has been shown to be compatible with CIGS chemistry and the relatively high temperature of the CIGS deposition, it has some disadvantages. It is more expensive than other metals that are better conductors (aluminum or copper for example), and it is not a good reflector in the spectral region of the maximum solar output. Thus light that does not create electron-hole pairs in the CIGS absorber on its first transit is not efficiently reflected back through the absorber for a second chance at causing a photoelectric event. Light that is absorbed in the moly, including the part of the solar spectrum that falls outside of the CIGS absorption band, only contributes to heating of the cell, which lowers its overall conversion efficiency. A better back electrode material is desirable in a large-scale manufacturing system.
Layer 3 is the CIGS p-type semiconductor absorber layer. It is usually about 2 to 3 microns thick, but could be somewhat thinner and attain the same or improved efficiency if the reflection of the back electrode layer (2) were improved. It would be extremely desirable to produce this layer by magnetron sputtering. This would enable a large-scale manufacturing process because magnetrons can readily be made in large sizes, and thickness and composition control can be excellent. A major provision of this invention is to demonstrate how this can be done with CIGS materials. Layer 4 is the n-type semiconductor layer that completes the formation of the p-n junction. It is much thinner than the absorber layer (about 0.05 microns), and it should be highly transparent to the solar radiation. Traditionally, it has been called the window layer, since it lets the light pass down to the absorber layer. It is also referred to as a buffer layer because it seems to help protect the p-n junction from damage induced by the deposition of the next layer. So far the use of CdS has resulted in the highest efficiency cells for the CIGS type absorber materials. But CdS is environmentally toxic, and it is difficult to deposit uniformly in large-scale by either the chemical bath method or by conventional RF magnetron sputtering. In addition, CdS is not highly transparent to the green and blue region of the solar spectrum, which makes it less compatible with higher band gap absorber layers.
At the 26th IEEE Photovoltaic Specialists Conference in October of 1977 Ullal, Zweibel, and von Roedern suggested a list of fifteen non-cadmium containing n-type materials that might be used as substitutes for the CdS layer. Of those materials SnO2, ZnO, ZrO2, and doped ZnO, are readily deposited by ordinary reactive magnetron sputtering of the metal in an argon and oxygen atmosphere. The reactive sputtering method that uses dual cylindrical rotary magnetrons as taught in U.S. Pat. No. 6,365,010 ('010) is especially useful for depositing these oxide layers. However, the dual cylindrical rotary magnetron technology can easily be extended to the reactive sputtering of sulfides and selenides, if facility improvements are made to handle the delivery of small amounts of the hydrogen sulfide and hydrogen selenide gases to the reactive deposition region. Using this technique two of the other materials on the list, ZnS and ZnSe, can be readily deposited with the dual cylindrical rotary magnetron system in the reactive mode. ZnS deposited by other methods has already been used instead of CdS in a laboratory demonstration cell that achieved a conversion efficiency of 18%. In addition, both ZnS and ZnSe have larger band gaps than CdS, so they are more efficient window materials. The less desirable method of conventional RF sputtering would work marginally for depositing thin layers of any remaining materials that cannot be readily formed into conducting targets.
Layer 5 is the top transparent electrode, which completes the functioning cell. This layer needs to be both highly conductive and as transparent as possible to solar radiation. ZnO has been the traditional material used with CIGS, but indium tin oxide (ITO), Al doped ZnO, and a few other materials could perform as well. Layer 6 is the antireflection (AR) coating, which can allow a significant amount of extra light into the cell. Depending on the intended use of the cell, it might be deposited directly on the top conductor (as illustrated), or on a separate cover glass, or both. For space-based power it is desirable to eliminate the cover glass, which adds significantly to expensive launch weight. Ideally, the AR coating would reduce the reflection of the cell to very near zero over the spectral region that photoelectric absorption occurs, and at the same time increase the reflection in the other spectral regions to reduce heating. Simple AR coatings do not adequately cover the relatively broad spectral absorption region of a solar cell, so multiple layer designs that are more expensive must be used to do the job more efficiently. Coatings that both perform the AR function and increase the reflection of unwanted radiation require even more layers and significant coating system sophistication. Aguilera et al in U.S. Pat. No. 6,107,564 issued 22 Aug. 2000 thoroughly review the prior art, and offer some improved AR coating designs for solar cell covers.
As previously mentioned the moly back contact layer is not a good reflector, nevertheless it has become the standard for thin-film type solar cells. Finding a better reflecting material that would otherwise withstand the processing conditions could improve the cell performance. The task is not simple. The back layer simultaneously should be a good conductor, be able to withstand high processing temperatures, and it should be a good reflector. Many metals in the periodic table meet at least one of these requirements, and any metal could be made thick enough to provide enough conductivity to function as the back electrical contact. The requirement of high processing temperatures eliminates the low melting point metals from consideration. Metals like tin, lead, indium, zinc, bismuth, and a few others melt at temperatures below the processing temperature for the CIGS or most other solar absorber materials. The motivation to lower the cost of the cell excludes metals like gold, platinum, palladium, rhodium, ruthenium, iridium, and osmium which otherwise have good conduction and reasonable reflection properties. With the exception of magnesium, which is highly reactive, all of the rest of the metals on the left half of the periodic chart of the elements are relatively poor reflectors, including molybdenum. The remaining candidates include aluminum, copper, silver, and nickel, and only nickel (and to a lesser extent molybdenum) resists forming insulating and poorly reflecting selenium compounds at the CIGS interface. However, nickel will severely degrade CIGS material if it is allowed to diffuse into it.
It is desirable to improve the large-scale manufacturability of thin-film solar cells in order to reduce their cost and make them competitive with conventional sources of electrical power generation. The use of the term large-scale in the context of the present invention implies the coating of either discrete substrates or continuous webs that have width dimensions of about a meter or more. This invention provides an apparatus and a method for sputter depositing all of the layers in the solar cell, and particularly the CIGS layer, which greatly increases the deposition area over which the required properties of the material can be achieved and controlled. It also provides improvements to the back contact/reflecting layer and the elimination of cadmium from the process.